Scientists trying to find out if British sheep have been infected by BSE have mistakenly been testing the brains of cows instead.

The Department for Environment, Food and Rural Affairs (Defra) has admitted that testing on the wrong tissue had been carried out for the past five years.

The extraordinary blunder was discovered after DNA checks, and means consumers will have to wait still longer for the clearest evidence that sheep meat is safe to eat - as most experts currently believe.

Ministers said last month that the UK's 20 million sheep and 20 million lambs might have to be destroyed if initial findings of BSE in sheep were confirmed.

"Theoretically", BSE could pass from cows to sheep

Scientists had expressed fears that the brain-wasting cattle illness bovine spongiform encephalopathy (BSE) - or mad cow disease - might have been passed between cows and sheep and its symptoms confused with a related disease, scrapie, which has been around for centuries but has never been shown to affect people.

It is also known that sheep would have been exposed to some of the same infected feed that passed BSE to cattle, before controls on animal feed were introduced in the late 1980s and early 1990s.

And laboratory research has shown sheep can be infected if injected with contaminated brain tissue from cattle. Brain and spinal tissue - known to be the main carrier of BSE infection - is already banned in sheep products as well as cattle.

Adequate safeguard

However, because of the different anatomy of sheep, infection is distributed more widely throughout the body, so this would not be an adequate safeguard.

This cross-checking has indeed raised doubts about the viability of the original sample

Lucian Hudson, Defra

Farmers insist it is highly unlikely that even if some sheep had contracted the disease, it would still be in the national flock.

On Thursday, the director of the communications directorate at the Department for Environment, Food and Rural Affairs (Defra) said that Veterinary Laboratories Agency tests on the brains of 180 sheep infected with transmissible spongiform encephalopathy (TSE) - a family of diseases that includes both BSE and scrapie - had shown no sign of BSE.

Lucian Hudson, director of the communications directorate at Defra, said: "Research has been under way for some time to see if it can be established whether BSE might have been present in the sheep flock in the early 1990s and masked as scrapie.

'Raised doubts'

"The work on material from the early 1990s has been conducted by the Institute for Animal Health (IAH). It was anticipated that some results would be ready to present to the Spongiform Encephalopathy Advisory Committee (Seac) on Friday.

"However, Defra, who commissioned the work, also commissioned cross-checking to guard against the possibility of material being contaminated by cattle brains - not least because it had been collected for entirely different experiments.

"This cross-checking has indeed raised doubts about the viability of the original sample."

Mr Hudson added: "The Seac chairman decided that Friday's meeting should be postponed.

"Defra has commissioned an independent scientific audit to establish the present status of the samples and will be working closely with the IAH to clarify the issue.

"This does not affect the Food Standards Agency's view, which is not to advise against the eating of sheep meat."